Inductive proximity sensor

ABSTRACT

The invention relates to an inductive proximity switch comprising an electrical resonant circuit and an energy source coupled herewith to compensate for the losses in the resonant circuit. The energy source is fully differentially coupled with the resonant circuit at the input side.

[0001] The invention relates to an inductive proximity sensor comprisingan electrical resonant circuit and an energy source coupled herewith tocompensate for losses in the resonant circuit.

[0002] Inductive proximity sensors are used in automation engineering todefine operating states in automating plants, production systems (e.g.welding robots) and process engineering plants. Proximity switcheseffect the detection of the presence or absence of electricallyconductive workpieces or machine parts. Applications such as revolutionmeasurements or speed measurements on rotating parts or parts moved in atranslatory manner are likewise conceivable.

[0003] Inductive proximity sensors have a lossy resonant circuit(oscillator) at the input side whose loss resistance can be changed, forexample, by the proximation of an electrically conductive medium. Ifsuch a response element is brought into sufficient proximity to theresonant circuit, a corresponding attenuation or attenuationequalization of the resonant circuit can be caused and detected inorder, for example, to trigger a switch signal.

[0004] The resonant circuit constantly loses energy, which has to becompensated to prevent an unwanted decaying of the oscillation, due tomagnetization losses in the ferrite, to the DC resistance of the coil orto the loss through the attenuation by the response element.

[0005] For this purpose, an energy source 40 is provided by which acompensation current I_(comp) in accordance with FIG. 2 can be fed intothe resonant circuit to compensate for the previously mentioned losses.

[0006] The energy source 40 is realized, for example, from avoltage-to-current converter 44 comprising a downstream or integratedcurrent limiter 46 in accordance with FIG. 3. The amplitude of theresonant circuit 42 can be set directly by the current limiter 46. Ifthe energy source 40 in accordance with FIG. 2 is subjected to positivefeedback, then it behaves like a negative, current limited resistanceR_(neg) since the voltage Uosc dropping over the resonant circuit andthe compensation current I_(comp) fed back are directed in opposingdirections to one another in accordance with the reference arrowconventions. The negative resistance R_(neg) is responsible for thecompensation of the losses in the resonant circuit.

[0007] Such LC resonant circuits 42 are frequently operated in thesaturated working range so that component tolerances only have aninsignificant influence on the response behavior of the resonant circuit42. This is shown in the region III of FIG. 4 in which the maximum valueGW of the compensation current I_(comp) is entered over the voltage lossU_(osc) at the resonant circuit. The resonant circuit 42 is excited inthis region III by rectangular current pulses. The same current isalways coupled into the resonant circuit 42 irrespective of the resonantcircuit amplitude due to the working range selected.

[0008] The resulting resonant circuit amplitudeU_(osc)=(4/π)×I_(sat)×R_(p) is only dependent on the saturation currentI_(sat) and the loss resistance R_(p) of the resonant circuit with asymmetrical output current (±I_(sat)) of the energy source 40.

[0009] The disadvantage of this working range, however, is a lowersensitivity on the attenuation of the resonant circuit. Electrically ormagnetically coupled interference, which can occur when the proximitysensor is used in the industrial environment, superimposes aninterference signal on the small wanted signal and this results in aninfluencing of the resonant circuit or to an accidental switch signal.

[0010] These interference signals can have both a push-pull characterand a push-push character. The influencing of the resonant circuit canultimately trigger an unwanted switch signal without an electricallyconductive target being disposed in the switch distance of the proximitysensor.

[0011] It is therefore an object of the invention to provide aninductive proximity sensor with a reduced proneness to interferencesignals and an improved response behavior.

[0012] This object is satisfied for a proximity sensor of the kind firstexplained in that the energy source is fully differentially coupled withthe resonant circuit at the input side.

[0013] In this connection, “fully differentially” means that the twoinputs of the energy source have no relation to the circuit ground (mostnegative potential of the circuit) or to any other constant potential.Only the push-push portion of the two inputs is controlled to signalground (typically half the operating voltage) to ensure the largestpossible excitation range. Unlike conventional energy sources with knownproximity sensors, none of the inputs of the energy source coupled withthe LC resonant circuit is on signal ground or circuit ground.

[0014] Those connections of the energy source are to be considered theinputs of the energy source via which the voltage dropping at theresonant circuit is picked up for the definition of the amplitude of theresonant circuit.

[0015] The proximity sensor has the advantage of a doubled signal strokedue to the coupling in accordance with the invention of the LC resonantcircuit and the energy source. The initially mentioned disadvantage of alower sensitivity due to the selected working range (see FIG. 4, regionIII) is thereby compensated with the advantage still being maintained ofa lower sensitivity of the response behavior of the resonant circuitwith respect to present component tolerances.

[0016] The coupling in accordance with the invention of the resonantcircuit and the energy source furthermore effects improved interferencesensitivity by a higher signal to noise performance ratio.

[0017] The proximity sensor in accordance with the invention furthermoreenables larger ranges with respect to known proximity sensors due to thedoubled signal stroke.

[0018] Ultimately, the proximity sensor in accordance with the inventiononly processes push-pull portions of the resonant circuit voltage andsuppresses push-push portions. Push-push portions in the resonantcircuit voltage can be caused, on the one hand, by inductive uncouplingsin the LC resonant circuit, caused for example by welding apparatuseslocated in the vicinity or by capacitive couplings into the connectionlines between the resonant circuit and the energy source. This push-pushinterference results in the conventional proximity sensor in aninfluencing of the resonant circuit and ultimately to a false switching.

[0019] It is preferred for the outputs of the energy source to becoupled with the inputs to make the compensation current available.

[0020] It must furthermore be noted with respect to the invention thatany kind of energy source can be used to compensate for the energylosses inside the resonant circuit. In particular, the energy source canhave a voltage-to-current converter, for example an operationaltransconductivity amplifier.

[0021] In a preferred embodiment of the invention, the compensationenergy, in particular the electrical compensation current, which can befed into the resonant circuit from the energy source, is limited by alimit switch. The amplitude of the resonant circuit can becorrespondingly limited in this manner. This limitation of thecompensation current or of the amplitude of the oscillation canpreferably be adjusted.

[0022] It is furthermore preferred if the energy source is produced inthe CMOS design.

[0023] Further embodiments of the invention are set forth in thedependent claims.

[0024] The invention will be explained in the following by way ofexample with reference to the drawings, in which are shown:

[0025]FIG. 1 a block diagram of a resonant circuit and of an energysource coupled herewith;

[0026]FIG. 2 a block diagram of a resonant circuit and of an energysource coupled herewith in accordance with the prior art;

[0027]FIG. 3 an embodiment of the energy source of the prior art; and

[0028]FIG. 4 a representation of the working ranges of a resonantcircuit.

[0029]FIG. 1 shows a resonant circuit formed by a capacitor C and a coilL having a loss resistance R_(p). The voltage U_(osc) drops over thisresonant circuit.

[0030] An energy source 10, which can be made as an operationaltransconductivity amplifier (OTA), is coupled with the resonant circuit.This coupling is made fully differentially, i.e. a positive input 11 anda negative input 13 of the energy source 10 are connected to theresonant circuit without the potential of one of these two connectionsbeing fixed to a constant value. The energy source 10 detects push-pullsignals of the resonant circuit via these two inputs 11, 13, whereasinterference signals, which are expressed due to the fully differentialcoupling as push-push signals, are suppressed.

[0031] To compensate the attenuation losses within the circuit,compensation currents I_(comp) are supplied to the resonant circuit viaoutputs 15, 17 of the energy source 10, and indeed in dependence on thepush-pull signal (resonance oscillation) applied to the inputs 11, 13.

[0032] As indicated in FIG. 1 for the saturation current I_(sat) of theenergy source 10, the limitation of the compensation currents I_(comp)is adjustable in order to thereby limit the maximum amplitude of theoscillation within the resonant circuit.

[0033] The circuit in accordance with FIG. 1 forms part of an inductiveproximity sensor which furthermore has an evaluation circuit (not shown)coupled with the resonant circuit. A proximation of the response elementis detected in a known manner by this in order, for example, to triggera switching signal—when the sensor is made as a proximity switch.

Reference Numeral List

[0034]10 energy source

[0035]11, 13 inputs of the energy source

[0036]15, 17 outputs of the energy source

[0037]40 energy source

[0038]42 resonant circuit

[0039]44 voltage-to-current converter

[0040]46 current limiter

[0041] L coil

[0042] C capacitor

[0043] R_(p) loss resistance

[0044] I_(comp) compensation current

[0045] l_(sat) saturation current

[0046] R_(neg) negative resistance

[0047] U_(osc) voltage drop

1. An inductive proximity sensor comprising an electrical resonant circuit (L, C, R_(p)) and an energy source (10) coupled herewith to compensate for the losses in the resonant circuit, characterized in that the energy source (10) is fully differentially coupled with the resonant circuit (L, C, R_(p)).
 2. A proximity sensor in accordance with claim 1, characterized in that none of the inputs of the energy source (10) coupled with the resonant circuit (L, C, R_(p)) lies at constant potential.
 3. A proximity sensor in accordance with claim 1, characterized in that the outputs (15, 17) are coupled with the inputs (11, 13) of the energy source (10).
 4. A proximity sensor in accordance with claim 1, characterized in that the energy source (10) has a voltage-to-current converter.
 5. A proximity sensor in accordance with claim 4, characterized in that the voltage-to-current converter is made as a transconductivity amplifier.
 6. A proximity sensor in accordance with claim 1, characterized in that the energy source can be current limited.
 7. A proximity sensor in accordance with claim 6, characterized in that the current limiter of the energy source (10) can be adjusted.
 8. A proximity sensor in accordance with claim 1, characterized in that the energy source (10) is made in a CMOS design.
 9. A proximity sensor in accordance with claim 1, characterized in that an evaluation circuit is provided for the detection of push-pull signals and is coupled with the resonant circuit. 